Axon degeneration occurs after nervous system injury and in neurodegenerative disease. Loss of axons and synapses results in neural circuit breakdown and is thought to be a primary factor driving functional loss in patients with neurological disorders. Despite its widespread importance in disease, remarkably little is known about the molecular mechanisms driving axon degeneration in any context. Studies of axon degeneration after axotomy (i.e. Wallerian degeneration) have proven an extremely useful approach to elucidate fundamental cellular events driving axon auto-destruction. For example, previous work on the mouse Wallerian degeneration slow molecule (WldS) revealed- surprisingly-that under certain conditions distal severed axon fibers can survive and remain functionally intact for remarkably long periods of time (i.e. weeks after axotomy) in the absence of a cell body. The long-term survival observed in WldS expressing axons raised the intriguing possibility that axon degeneration might be an active process of axon auto-destruction, akin to apoptotic cell death. In our first funding cycle we developed the first Drosophila models for Wallerian degeneration and demonstrated that severed fly axons undergo Wallerian degeneration that can be potently suppressed by the mouse WldS molecule. These data indicated the molecular mechanisms of Wallerian degeneration are an ancient, and conserved feature of neuronal cell types. Inspired by WldS we performed the first forward genetic screen for Drosophila mutations that suppressed Wallerian degeneration. Strikingly, we found that loss of the kinase adaptor molecule dSarm (sterile alpha/Armadillo/Toll-Interleukin receptor homology domain protein) suppressed Wallerian degeneration for the lifespan of the fly (>3 weeks). We also made the exciting discovery that this pathway was functionally conserved in mammals: Sarm1-/- mice exhibited robust suppression of Wallerian degeneration both in vivo and in vitro. This data identifies dSarm/Sarm1 as a conserved axon death gene, whose endogenous function is to promote axon degeneration after axotomy. In Aims 1 and 2 of this proposal we will use Drosophila to study the cellular and molecular mechanisms by which dSarm promotes axonal degeneration. In Aim 3 we will characterize a collection of novel Drosophila mutants that we have recently isolated that also potently suppress axon death like dsarm. These studies are at the heart of our long-term, comprehensive effort to understand how axons destroy themselves after injury. We expect our findings to have a major impact on our understanding of axon degeneration after injury or in human disease, and the novel molecules we identify will be excellent candidates for therapeutic intervention in human neurological disorders involving axonal and synaptic loss.